The modern trend in the construction of buildings is to make them more and more air tight, in order to be able to achieve better control over the interior environment, particularly the temperature and humidity thereof. However, utilisation of oxygen by inhabitants of the building and other factors rendering the air stale, such as noxious gases given off by furnishing or odours generated in kitchens or bathrooms, usually necessitate the building being provided with some form of ventilation, preferably forced ventilation. If stale air is to be evacuated from the building, it must be replaced by fresh air coming in from the exterior. In those regions of the world experiencing cold winters, such incoming fresh air is often much colder than the stale air being evacuated, a fact that places an added burden on the building's heating system. The need to heat the incoming air up to the interior temperature may involve heating it as much as 20.degree. to 40.degree. C. Evacuation of the stale air at a relatively elevated temperature clearly represents a major waste of heat.
For these reasons, heat exchangers have been developed for the purpose of transferring at least some of the heat in the stale air as it leaves the building to the colder incoming fresh air. This diminishes the extent to which further heat needs to be supplied to the fresh air by the heating system of the building and generally renders the entire heating operation more efficient.
In this connection, it should be explained that the term "building" will be used herein for convenience to refer to any space that is required to be force ventilated, whether it be a large multi-unit building, an individual apartment or other subdivision of a larger building, a house, warehouse, shop, store or even a moving structure such as a ship, train or the like. The present invention is applicable to any such structures that are provided with forced ventilation and internal heating (or cooling) to maintain a temperature difference between the interior and the exterior. The foregoing reference to cooling envisages use of a heat exchanger according to the present invention to employ air evacuated from an air conditioned space to precool hotter incoming air.